1. Field of the Invention
The present invention relates to a lead frame and a semiconductor device. More specifically, the invention relates to a lead frame and a semiconductor device adapted to preventing  prevent package cracking the heat  as a result of the heat generated at the time of reflow soldering.
2. Description of the Prior Art
Plastic molded semiconductor devices of the surface mounted type which permits the leads to be directly soldered onto the substrate are becoming established as a standard substitute for conventional devices of the pin inserted type. If packages of such a type are preserved in a high-temperature and high-humidity environment, the plastic will in time absorb moisture which will then be vaporized in an interface between a chip pad (a portion where the chip is mounted which hereinafter is referred to as chip pad) and the plastic portion at the time of heating for soldering (reflow), giving rise to the formation of cracks in the lower corner surface of the chip pad. The cracks develop at the time of reflowing the solder and are usually called reflow cracks.
According to a conventional technique for preventing the reflow cracks from occurring, the back surface of the package is perforated for allowing any vapor generated to escape as is disclosed in Japanese Patent Laid-Open No. 208847/1985.
In order to increase the adhering strength on the interface between the plastic portion and the chip pad while preventing the formation of a gap, furthermore, a method has been proposed according to which the surface of the chip pad opposite to the surface on which the chip is mounted is made rugged as disclosed in Japanese Patent Laid-Open Nos. 199548/1983 and 186044/1965. A technique for perforating a portion that corresponds to the chip pad has also been disclosed in Japanese Patent Laid-Open No. 16357/1984 and in U.S. Pat. No. 4,633,583.
Among the above-mentioned conventional techniques, the method of perforating the lower surface of the package helps prevent the formation of reflow cracks but forms a passage that allows the moisture to flow between the exterior and interior of the package which then results in the corrosion of chip electrodes.
The method which forms a rugged surface on the surface of the chip pad opposite to the chip-mounting surface is effective for preventing the displacement between the chip pad and the adhering surface of the plastic portion, but is not effective for preventing the displacement in a direction in which they will be separated away from each other since the plastic portion easily escapes from the recessed portion.
The moisture contained in the plastic vaporizes at the time of reflow soldering of the plastic molded semiconductor device, and the vapor pressure acts on the voids in the interface between the chip pad and the plastic or acts on the cavities in the non-adhered portions to promote the peelng  peeling on the interface between the chip pad and the plastic. Even if the cavity becomes progressively larger as a result of peeling, the ambient water content is supplied thereto by diffusion. Therefore, the pressure in the cavity does not decrease, and the plastic portion undergoes deformation giving rise to the formation of cracks starting from a portion where a maximum stress generates at the end of the chip pad (see crack 10 in FIG. 6). According to the above-mentioned Japanese Patent Laid-Open No. 16357/1984, part of the chip pad is removed and plastic is filled in this portion to prevent the peeling by the thermal stress. Moreover, since the thickness of the plastic portion increases equivalently, resistance against the humidity can be improved to some extent. However, stress in the portion where a maximum stress develops is little different from the case of when a portion of the chip pad is not removed because of the deformation that develops when the plastic portion is peeled off from the chip pad being caused by the vapor pressure at the time of reflow soldering. Therefore, this structure is not very effective for coping with the cracks that develop in the plastic portion at the time of reflow soldering.
In order to prevent the chip from breaking at the time of die bonding or in the subsequent temperature aging, it had nitherto  hitherto been attempted to use a lead frame material having a small coefficient of expansion or to use a die bonding agent having a small coefficient of elasticity. However, the traditional methods limited the range for selecting the materials, pushed up the manufacturing cost, and were not completely effective for suppressing the reflow cracks.
The dimple processing is effective for preventing the development of cracks in the plastic portion at the lower end of the chip pad under the temperature cycle testing. However, when there exists a large difference in the linear thermal expansion coefficient between the lead frame material and the plastic material, the cracks easily develop at the root of the resinous protrusion filled in the dimples.
According to the performation  perforation method which forms apertures that reach from the surface of the package to the side of the chip pad opposite to the chip mounting side  surface in order to suppress the reflow cracks, the moisture easily reaches the interface where the chip pad and the plastic part are adhered together and further reaches the surface of the chip through apertures. When used for extended periods of time, therefore, the device becomes defective as the aluminum (A1  Al) wiring is corroded. Further, this method is not quite effective for suppressing the development of cracks in the plastic at the lower end of the chip pad caused by the temperature cycle.
The  An object of the present invention is to prevent the reflow cracks resulting from vapor pressure.
Another object of the present invention is to provide semiconductor device which permits minimum destruction of the semiconductor chips at the time of die bonding or in the subsequent temperature aging, which prevents cracks from developing in the plastic at the lower end of the chip pad even under the temperature cycling test, and which exhibits increased resistance against the reflow cracks.
The above object is  objects are achieved by forming a through hole of a particular shape in the chip pad, and holding a plastic portion on the chip pad utilizing the hole, to thereby decrease the stress that generates in the plastic portion on the lower corner surface of the chip pad where reflow cracks may develop.
To prevent the cracks from developing in the plastic at the time of reflow,  soldering, it is essential that excessive stress is not generated at an end of the chip pad where maximum stress is generated even when the plastic portion is peeled off from the chip pad. This requirement is achieved by a structure which is capable of preventing the plastic portion from deforming even when there exists no adhesive force between the plastic portion and the chip pad, i.e., by a structure which does not permit the plastic portion to escape  be deformed away from the chip pad irrespective of the vapor pressure.
A first invention  inventive aspect of the present application is concerned with a unitary structure including a lead frame consisting of a chip pad for mounting a semiconductor chip and a group of leads connected to said chip pad along the periphery thereof , the improvement wherein a through hole is formed in at least one place of the chip pad, said  the through hole having a tilted portion (tapered portion) with respect to the chip-mounting surface of the chip pad.
The through hole may assume the form in which the entire hole is tilted relative to the direction of thickness of the chip pad or be in the form in which the hole as a wedge portion in the thickness of the chip pad, or may assume any other form.
A second inventive aspect of the present application  invention is concerned with a unitary structure including a lead frame consisting of a chip pad for mounting a semiconductor element  chip and a group of leads connected to said chip pad along the periphery thereof , the improvement wherein a through hole is formed in at least one location of the chip pad, said  the through hole having an opening on  end disposed in the chip-mounting side corresponding to  surface and having an area that is greater than an area of a hole opening end thereof on the side opposite to the chip-mounting side  surface.
A third inventive aspect of the present application  invention is concerned with a semiconductor device which comprises a semiconductor chip, a unitary structure of a chip pad for mounting the semiconductor chip and a group of leads connected to the chip pad along the periphery thereof , and a plastic portion for sealing inner lead portions in  of the group of leads, the chip pad and the semiconductor chip, wherein a through hole is formed in at least one portion of the chip pad, said through hole having a portion that is tilted relative to the chip-mounting surface of the chip pad.
The through hole may assume the form in which the entire hole is tilted relative to the direction of thickness of the chip pad, the form in which a wedge portion is formed in the direction of  thickness of the chip pad, the form in which the area of the hole opening on  end in the chip-mounting side  surface is greater than the area of the hole opening on  end in the side  surface opposite to the chip-mounting side  surface, or it may assume another form.
Throughout the first to third mentioned inventive aspects of the present application, it is desired that the through hole has an  a first opening end area on  in the chip-mounting side  surface which lies from 24% of the area of the chip pad on the side of the chip-mounting surface through up to  is less than 80% of the junction area between the chip pad and the chip. It is further desired to form a groove that surrounds the through hole on the side of  opening end in the chip-mounting surface of the chip pad.
Furthermore, the above-mentioned objects are also achieved by using a lead frame in which the chip pad is comprised of an adhesive-mounting portion, a chip support portion and a lead for connecting both of them together, and by adhering the chip pad and the semiconductor chip together in the adhesive-mounting portion only.
A fourth inventive aspect of the present application  invention is concerned with a semiconductor device which comprises a semiconductor chip, a chip pad for mounting the semiconductor chip, and a group of leads that include a chip pad-hanging  pad supporting lead (chip-mounting portion, i.e.,  a lead-like support member for supporting the chip pad which is hereinafter referred to as chip pad-hanging  pad supporting lead) that is linked to the chip pad, wherein the chip pad is divided into a  has an outer peripheral annular  portion and a central island portion, the island portion is connected to a portion  part of the annular  peripheral portion as a unitary structure so that it  the island portion is supported by the annular  peripheral portion, and the semiconductor elment  chip is indirectly mounted on the island  chip portion via an adhesive and is further mounted on the element-mounting  chip mounting surface of the annular  peripheral portion directly or via a gap (space).
A fifth inventive aspect of the present application  invention is concerned with a semiconductor device having a component structure similar to that of the fourth invention, however  inventive aspect. However, the chip pad is divided into an adhesive-applied  adhesive application portion (i.e., island portion) for adhering semiconductor chip and other portions (annular  peripheral portion, lead portion, etc.).
In the invention according  According to the present invention, it through hole be  is desired that the chip pad is  through hole has a portion tapered in the direction of its thickness such that the area of the chip pad on the chip side  opening end of the through hole in the chip-mounting surface thereof, as affected by the size of the through hole opening(s)  is smaller  greater than the through hole opening area of the chip pad surface on the side opposite to the chip side  chip-mounting surface. It is further desired to form a groove along the periphery of the island portion to prevent the bonding agent  adhesive from flowing out.
Preferably, furthermore, the island portion should be lower than the annular  outer peripheral portion on the chip-mounting side. In other words, it is desired that the adhesive-mounting  adhesive application portion has a surface that is lower than the surface of the chip support portion so as to form a step therebetween.
It is further desired that a part of the surface of the annular  outer peripheral portion on the chip-mounting surface of the chip pad be recessed relative to the remaining surface or surfaces of the annular  outer peripheral portion on the chip-mounting surface. In other words, it is desired to form a dent in a portion for supporting the chip in the chip support portion of the chip pad.
The island portion may not necessarily be only one portion but may be divided into a plurality of portions. It is further desired that the island portion has a thickness smaller than that of other lead portions.
The aforementioned modes  features may be suitably combined together as a matter of course.
To roughly find the stress in the plastic portion on the lower corner surface of the chip pad where the reflow cracks develop, the plastic portion under the chip pad should be modeled in the form of an elongated  a rectangular flat plate which has defined  fixed peripheral sides and on which the pressure is uniformly distributed as shown in FIG. 5. In this case, a maximum stress is generated at the center of the long side and is given by the following equation.                     σ        =                              β            ·                                          a                2                                            h                2                                              ⁢                      xe2x80x83                    ⁢          p                                    (        1        )            
where xcex2 denotes a coefficient of stress determined by the ratio of a long side to a short side, a denotes the length of the short side, h denotes the plate thickness, and p denotes a pressure of the water vapor.
As will be obvious from the equation (1), the stress that is generated increases in proportion to the square power of the chip pad size a. As the chip size increases, therefore, the reflow crack (designated at 10 in FIG. 6) tends to develop easily. To decrease the stress, therefore, the length of the short side should be shortened or the plate thickness should be increased. However, increase in the plate thickness results in the increase in the thickness of the package which is not adapted to flat packages that feature reduced thicknesses. Moreover, the chip pad size is not allowed to become smaller than the size of the chip, and is thus determined by the size of the chip.
According to the present invention, therefore, a plastic-holding portion is provided on a portion of the chip pad to divide the peeling portion of the chip pad. This helps substantially decrease the chip pad size a, and  whereby stress in the plastic portion decreases and the reflow cracks are prevented from developing in the resin  plastic portion.
According to the present invention, therefore, the distance for holding the plastic portion under the chip pad becomes short when the chip pad and the plastic portion are peeled off from each other, and  whereby reduced stress develops in the resin  plastic portion due to vapor pressure and reflow cracks are prevented from developing.
According to one aspect of the present invention, the chip and the chip pad are adhered together only at a central portion of the chip, and the stress that is generated in the chip is nearly equal to a value that results when the chip of a length that corresponds to the adhered portion is adhered over its entire mounting surface. Therefore, even those chips having large sizes do not develop cracks.
As for cracks that may develop in the resin  plastic portion under the lower end  below edge of the chip pad through  under the temperature cycle testing,  cycling since there exists a gap between the annular portion of chip pad and the island portion, and  the resin  plastic material introduced into this gap eliminates relative slippage between the plastic and the surface of the chip pad of the side opposite to the chip. Therefore, the stress is suppressed from developing at the lower portion of the chip pad, and the crack is prevented from developing in the plastic portion.
The coefficient of linear expansion of the plastic material is greater than that of the chip pad. Under a high-temperature condition at the time of reflow solderin  soldering, therefore, the plastic of a  portion where  in the hole is formed  is pushed onto the side  inner peripheral surface of the chip pad  hole. Moreover, since the side  inner peripheral surface of the chip pad  hole has been made rugged at the time when it was formed, the side  inner peripheral surface of the chip pad  hole prevents the plastic from swelling toward the lower side of the chip pad even in the case where the adhering force is insufficient between the plastic and the chip and, further, even when the vapor pressure is applied thereto. Here, the side surface of the island portion works as a fixed fulcrum. Therefore, the stress being  generated is less than that of  in the conventional chip pad  semiconductor device, and the  reflow cracks hardly develop.
As described above, the present invention provides a semiconductor device which prevents the chips from being destroyed at the time of die bonding and in the subsequent temperature aging, reduces the propensity for the formation of cracks in the plastic portion at the lower end  edge of the chip pad through  under the temperature cycle testing becomes minimized  cycling, and exhibits excellent resistance to the formation of reflow cracks becomes realizable .
According to the present invention, therefore, there is obtained a semiconductor device which does not permit semiconductor chips to be destroyed at the time of die bonding and in the subsequent temperature aging, the formation of cracks in the plastic portion at the lower end of the chip pad through the temperature cycle testing becomes minimized , and provides excellent resistance to the formation of reflow cracks becomes realizable .